1. Field of the Invention
This invention relates to a braking method, a braking circuit and a speed control apparatus for a DC motor which is pulse-driven, and more particularly to a braking method, a circuit therefor and control apparatus for a DC motor which is capable of plugging without requiring an external braking current.
2. Description of the Prior Art
A pulse driving method has been well known as one drive control method for a DC motor, which is advantageous in that its drive circuit has less electrical losses and is controllable by use of a digital signal from a computer or the like. This method generally employs a servo system for detecting rotation of the motor by a rotary encoder or the like so as to control the rotational speed of the motor so as to be a set value on the basis of an output of the encoder. In other words, when the rotational speed of the motor is smaller than the set value, a duty ratio of the driving pulse is enlarged so as to increase a mean value of voltage fed to the DC motor, thereby raising the rotational speed thereof. On the other hand, when the rotational speed of the DC motor exceeds the set value, some method is used to apply braking to the DC motor.
Now, the braking method for the motor and a rotary system driven thereby, is roughly sorted into the mechanical method and electrical method.
The electrical method is different from the mechanical one in that it has the advantage of avoiding wear, heating and damage of the braking.
There are three typical methods for electrically braking the motor.
With dynamic braking, the motor is operated as a generator by cutting off the power supply to the running motor, and the kinetic energy of the motor, which is rotating by inertia, is converted into electrical energy, and the electrical energy is fed to a resistance, thereby being converted into thermal energy and dispersed.
With regenerative braking, the kinetic energy of a rotor, in the same fashion as the aforesaid dynamic braking, is converted into electrical energy which is then returned to a power source, thereby being fed to other loads.
With plugging a current flows in the winding of motor in the reverse direction to that when the motor is driven, thereby generating a torque in a direction which is opposite to the rotation direction of the motor. In other words, the motor is driven so as to rotate reversely, thereby being braked.
The above methods are properly selectively used in dependance upon the purpose of using the motor. The present invention applies the plugging method, but in a case of controlling the rotational speed of the motor by plugging, a braking current should be externally supplied and also of the supplied current should be strictly controlled, whereby the plugging method is defective in that a special apparatus for strict current control is required.